In wireless communication systems, such as defined by 3GPP Long Term Evolution (LTE/LTE-A) specification, user equipments (UE) and base stations (eNodeB) communicate with each other by sending and receiving data carried in radio signals according to a predefined radio frame format. Typically, the radio frame format contains a sequence of radio frames, each radio frame having the same frame length with the same number of subframes. The subframes are configures to perform uplink (UL) transmission or downlink (DL) reception in different Duplexing methods. Time-division duplex (TDD) is the application of time-division multiplexing to separate transmitting and receiving radio signals. TDD has a strong advantage in the case where there is asymmetry of the uplink and downlink data rates. Seven different TDD configurations are provided in LTE/LTE-A systems to support different DL/UL traffic ratios for different frequency bands.
FIG. 1 (Prior Art) illustrates the TDD mode UL-DL configurations in an LTE/LTE-A system. FIG. 1 shows that each radio frame contains ten subframes, D indicates a DL subframe, U indicates an UL subframe, and S indicates a Special subframe/Switch point (SP). Each SP contains a DwPTS (Downlink pilot time slot), a GP (Guard Period), and an UpPTS (Uplink pilot time slot). DwPTS is used for normal downlink transmission and UpPTS is used for uplink channel sounding and random access. DwPTS and UpPTS are separated by GP, which is used for switching from DL to UL transmission. The length of GP needs to be large enough to allow the UE to switch to the timing advanced uplink transmission. These allocations can provide 40% to 90% DL subframes.
In 3GPP LTE Rel-11 and after, the trend of the system design shows the requirements on more flexible configuration in the network system. Based on the system load, traffic type, traffic pattern and so on, the system can dynamically adjust its parameters to further utilize the radio resource and to save the energy. One example is the support of dynamic TDD configuration, where the TDD configuration in the system may dynamically change adapting to the DL-UL traffic ratio. When the change better matches the instantaneous traffic situation, the system throughput will be enhanced.
In traditional TDD systems, UL-DL configuration is broadcasted in the system information, i.e. SIB1. The mechanism for adapting UL-DL allocation is based on the system information change procedure. The semi-static allocation may or may not match the instantaneous traffic situation. In adaptive TDD systems, the notification of TDD change may be sent through a dedicated signaling, i.e., Radio Resource Control (RRC), Media Access Control (MAC) or Physical Downlink Control Channel (PDCCH) signaling, where the change period may be much less than the change of SIB1 (640 ms). The benefits to adopt TDD configuration change by dedicated signaling is that it can be adjusted more efficiently and frequently to match the instantaneous traffic pattern.
In order to enable reasonable UE battery consumption, discontinuous reception (DRX) in E-UTRAN is defined. The UE may be configured via RRC with a DRX functionality that controls the UE's PDCCH monitoring activity for the UE's C-RNTI, TPC-PUCCH-RNTI, TPC-PUSCH-RNTI and Semi-Persistent Scheduling C-RNTI (if configured). When in RRC CONNECTED mode, if DRX is configured, the UE is allowed to monitor the PDCCH discontinuously using the DRX operation. Otherwise, the UE monitors the PDCCH continuously. The DRX parameters are configured by eNodeB, a trade-off between UE battery saving and latency reduction of data transmission.
FIG. 2 (Prior Art) illustrates a DRX Cycle where the periodic repetition of the On Duration followed by a possible period of inactivity. The following definitions may apply to DRX in E-UTRAN: 1) on-duration: a duration in downlink subframes that the UE waits for, after waking up from DRX, to receive PDCCHs. If the UE successfully decodes a PDCCH, the UE stays awake and starts the inactivity timer; 2) inactivity-timer: a duration in downlink subframes that the UE waits to successfully decode a PDCCH, from the last successful decoding of a PDCCH, failing which it re-enters DRX. The UE shall restart the inactivity timer following a single successful decoding of a PDCCH for a first transmission only (i.e. not for retransmissions); 3) active-time: the total duration that the UE is awake. This includes the “on-duration” of the DRX cycle, the time UE is performing continuous reception while the inactivity timer has not expired and the time UE is performing continuous reception while waiting for a DL retransmission after one HARQ RTT. Based on the above, the minimum active time is of length equal to on-duration, and the maximum is undefined.
More specifically, when a DRX cycle is configured, the Active Time includes the time while 1) onDurationTimer or drx-InactivityTimer or drx-RetransmissionTimer or mac-ContentionResolutionTimer is running; 2) a Scheduling Request is sent on PUCCH and is pending; 3) an uplink grant for a pending HARQ retransmission occurs and there is data in the corresponding HARQ buffer; or 4) a PDCCH indicating a new transmission addressed to the C-RNTI of the UE has not been received after successful reception of a Random Access Response for the preamble not selected by the UE. The DRX timers and HARQ RTT timer are all counted in PDCCH subframe, which may cause problems in adaptive TDD systems because frequent TDD configuration changes may lead to non-alignment counting in PDCCH subframe in eNodeB and UE sides. DRX operations in adaptive TDD systems thus need to be enhanced.